3D chip with shared clock distribution network

ABSTRACT

Some embodiments of the invention provide a three-dimensional (3D) circuit that is formed by stacking two or more integrated circuit (IC) dies to at least partially overlap and to share one or more interconnect layers that distribute power, clock and/or data-bus signals. The shared interconnect layers include interconnect segments that carry power, clock and/or data-bus signals. In some embodiments, the shared interconnect layers are higher level interconnect layers (e.g., the top interconnect layer of each IC die). In some embodiments, the stacked IC dies of the 3D circuit include first and second IC dies. The first die includes a first semiconductor substrate and a first set of interconnect layers defined above the first semiconductor substrate. Similarly, the second IC die includes a second semiconductor substrate and a second set of interconnect layers defined above the second semiconductor substrate. As further described below, the first and second dies in some embodiments are placed in a face-to-face arrangement (e.g., a vertically stacked arrangement) that has the first and second set of interconnect layers facing each other. In some embodiments, a subset of one or more interconnect layers of the second set interconnect layers of the second die has interconnect wiring that carries power, clock and/or data-bus signals that are supplied to the first IC die.

CLAIM OF BENEFIT TO PRIOR APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 16/889,698, filed Jun. 1, 2020, now published asU.S. Patent Publication 2020/0294858. U.S. patent application Ser. No.16/889,698 is a continuation application of U.S. patent application Ser.No. 15/976,817, filed May 10, 2018, now issued as U.S. Pat. No.10,672,663. U.S. patent application Ser. No. 15/976,817 claims thebenefit of U.S. Provisional Patent Application 62/619,910, filed Jan.21, 2018, U.S. Provisional Patent Application 62/575,184, filed Oct. 20,2017, U.S. Provisional Patent Application 62/575,259, filed Oct. 20,2017, U.S. Provisional Patent Application 62/575,240, filed Oct. 20,2017. U.S. patent application Ser. No. 15/976,817 is acontinuation-in-part of U.S. patent application Ser. No. 15/725,030,filed Oct. 4, 2017, now issued as U.S. Pat. No. 10,522,352. U.S. patentapplication Ser. No. 15/725,030 claims the benefit of U.S. ProvisionalPatent Application 62/405,833, filed Oct. 7, 2016. U.S. patentapplication Ser. No. 16/889,698, now published as U.S. PatentPublication 2020/0294858 and U.S. patent application Ser. No.15/976,817, now issued as U.S. Pat. No. 10,672,663 are incorporatedherein by reference.

BACKGROUND

Electronic circuits are commonly fabricated on a wafer of semiconductormaterial, such as silicon. A wafer with such electronic circuits istypically cut into numerous dies, with each die being referred to as anintegrated circuit (IC). Each die is housed in an IC case and iscommonly referred to as a microchip, “chip,” of IC chip. According toMoore's law (first proposed by Gordon Moore), the number of transistorsthat can be defined on an IC die will double approximately every twoyears. With advances in semiconductor fabrication processes, this lawhas held true for much of the past fifty years. However, in recentyears, the end of Moore's law has been prognosticated as we are reachingthe maximum number of transistors that can possibly be defined on asemiconductor substrate. Hence, there is a need in the art for otheradvances that would allow more transistors to be defined in an IC chip.

BRIEF SUMMARY

Some embodiments of the invention provide a three-dimensional (3D)circuit that is formed by stacking two or more integrated circuit (IC)dies to at least partially overlap and to share one or more interconnectlayers that distribute power, clock and/or data-bus signals. The sharedinterconnect layers include interconnect segments (also referred to asinterconnect lines or wires) that carry power, clock and/or data-bussignals. In some embodiments, the shared interconnect layers are higherlevel interconnect layers (e.g., the top interconnect layer of each ICdie).

In some embodiments, the stacked IC dies of the 3D circuit include firstand second IC dies. The first die includes a first semiconductorsubstrate and a first set of interconnect layers defined above the firstsemiconductor substrate. Similarly, the second IC die includes a secondsemiconductor substrate and a second set of interconnect layers definedabove the second semiconductor substrate. As further described below,the first and second dies in some embodiments are placed in aface-to-face arrangement (e.g., a vertically stacked arrangement) thathas the first and second set of interconnect layers facing each other.In some embodiments, a subset of one or more interconnect layers of thesecond set interconnect layers of the second die has interconnect wiringthat carries power, clock and/or data-bus signals that are supplied tothe first IC die. This subset is referred to below as the sharedinterconnect-layer subset.

In some embodiments, numerous electronic components (e.g., activecomponents, like transistors and diodes, or passive components, likeresistors and capacitors) are defined on the first semiconductorsubstrate, and these electronic components are connected to each otherthrough interconnect wiring on the first set of interconnect layers toform numerous microcircuits (e.g., Boolean gates) and/or larger circuits(e.g., functional blocks). In some of these embodiments, the power,clock and/or data-bus signals from the shared interconnect-layer subsetof the second die are supplied to several electronic components,microcircuits, and larger circuits of the first die. Also, in some ofthese embodiments, the power, clock and/or data-bus signals from theshared interconnect-layer subset are also supplied to electroniccomponents, microcircuits, and larger circuits that are formed on thesecond substrate of the second die.

In some embodiments, the face-to-face arranged first and second dieshave their top interconnect layers bonded to each other through a directbonding process that establishes direct-contact metal-to-metal bonding,oxide bonding, or fusion bonding between these two sets of interconnectlayers. An example of such bonding is copper-to-copper (Cu—Cu) metallicbonding between two copper conductors in direct contact. In someembodiments, the direct bonding is provided by a hybrid bondingtechnique such as DBI® (direct bond interconnect) technology, and othermetal bonding techniques (such as those offered by Invensas BondingTechnologies, Inc., an Xperi Corporation company, San Jose, Calif.).

The direct bonding techniques of some embodiments allow a large numberof direct connections (e.g., more than 1,000 connections/mm², 10,000connections/mm², 100,000 connections/mm², 1,000,000 connections/mm² orless, etc.) to be established between the top two interconnect layers ofthe first and second dies, in order to allow power, clock and/ordata-bus signals to traverse between the first and second IC dies. Theseconnections traverse the bonding layer between the two face-to-facemounted dies. When these connections provide signals from the topinterconnect layer of the second die to the top interconnect layer ofthe first die, the first die in some embodiments uses other ICstructures (e.g., vias) to carry these signals from its top interconnectlayer to other layers and/or substrate of the first die.

These connections between the top interconnect layers of the first andsecond IC dies are very short in length, which, as further describedbelow, allows the signals on these lines to reach their destinationsquickly while experiencing minimal capacitive load from other nearbywiring. In some embodiments, the pitch between two neighboringdirect-bonded connections (i.e., the distance between the centers of thetwo neighboring connections) that connect the top interconnect layers ofthe first and second dies can be extremely small, e.g., the pitch fortwo neighboring connections can be between 0.2 μm to 15 μm. This closeproximity allows for the large number and high density of suchconnections between the top interconnect layers of the first and seconddies. Moreover, the close proximity of these connections does notintroduce much capacitive load between two neighboring z-axisconnections because of their short length and small interconnect padsize.

In some embodiments, the top interconnect layers of the first and seconddies have preferred wiring directions that are orthogonal to each other.Specifically, the top interconnect layer of the first die has a firstpreferred routing direction, while the top interconnect layer of thesecond die has a second preferred routing direction. In someembodiments, the first and second preferred routing directions areorthogonal to each other, e.g., the top layer of one die has ahorizontal preferred routing direction while the top layer of the otherdie has a vertical preferred routing direction. In other embodiments,the top layer of the first die has the same preferred routing directionas the top layer of the second die, but one of the two dies is rotatedby 90 degrees before bonding the top two layers together through adirect bonding technique.

Having the wiring direction of the top interconnect layers of the firstand second dies be orthogonal to each other has several advantages. Itprovides better signal routing between the IC dies and avoids capacitivecoupling between long parallel segments on adjacent interconnect layersof the two dies. Also, it allows the top interconnect layers of thefirst and second dies to conjunctively define a power distributionnetwork (referred to as power mesh below) or a clock distributionnetwork (referred to below as clock tree) that requires orthogonal wiresegments in two different interconnect layers.

Orthogonal wiring directions on the top layers of the first and seconddies also increases the overlap between the wiring on these layers,which increases the number of candidate locations for bonding differentpairs of wires on the top interconnect layers of the different dies toprovide power signals and/or clock signals from one die to another die.For instance, in some embodiments, the first die has one set ofalternating power and ground lines that traverses along one direction(e.g., the horizontal direction), while the second die has another setof alternating power and ground lines that traverses along anotherdirection (e.g., the vertical direction). The power/ground lines on onedie's interconnect layer can be directly bonded to correspondingpower/ground lines on the other die's interconnect layer at each or someof the overlaps between corresponding pair of power lines.

This direct bonding creates a very robust power mesh for the first andsecond dies without using two different interconnect layers for each ofthese two dies. In other words, defining a power mesh by connectingorthogonal top interconnect layers of the first and second dies througha direct bonding scheme eliminates one or more of power layers in eachdie in some embodiments. Similarly, defining a clock tree by connectingorthogonal top interconnect layers of the first and second dies througha direct bonding scheme eliminates one or more of clock layers in eachdie in some embodiments. In other embodiments, the first die does nothave a power mesh or clock tree, as it shares the power mesh or clocktree that is defined in the interconnect layer(s) of the second die.

The first and second dies in some embodiments are not face-to-facestacked. For instance, in some embodiments, these two dies areface-to-back stacked (i.e., the set of interconnect layers of one die ismounted next to the backside of the semiconductor substrate of the otherdie), or back-to-back stacked (i.e., the backside of the semiconductorsubstrate of one die is mounted next to the backside of thesemiconductor substrate of the other die). In other embodiments, a thirddie is placed between the first and second dies, which are face-to-facestacked, face-to-back stacked (with the third die between the backsideof the substrate of one die and the set of interconnect layers of theother die), or back-to-back stacked (with the third die between thebacksides of the substrates of the first and second dies). While someembodiments use a direct bonding technique to establish connectionsbetween the top interconnect layers of two face-to-face stacked dies,other embodiments use alternative connection schemes (such as throughsilicon vias, TSVs, through-oxide vias, TOVs, or through-glass vias,TGVs) to establish connections between face-to-back dies and betweenback-to-back dies.

Stacking IC dies to share power, clock and/or data-bus signals betweentwo dies has several advantages. This stacking reduces the overallnumber of interconnect layers of the two dies because it allows the twodies to share some of the higher-level interconnect layers in order todistribute power, clock and/or data-bus signals. For example, asdescribed above, each die does not need to devote two interconnectlayers to create a power/ground mesh, because this mesh can be formed bydirect bonding the power/ground top interconnect layer of one die withthe power/ground top interconnect layer of the other die. Reducing thehigher-level interconnect layers is beneficial as the wiring on theselayers often consume more space due to their thicker, wider and coarserarrangements. In addition, the ability to share the use of theseinterconnect layers on multiple dies may reduce the congestion and routelimitations that may be more constrained on one die than another.

Stacking the IC dies in many cases also allows the wiring for deliveringthe power, clock and/or data-bus signals to be much shorter, as thestacking provides more candidate locations for shorter connectionsbetween power, clock and/or data-bus signal interconnects and thecircuit components that are to receive these signals. For instance,instead of routing data-bus signals on the first die about severalfunctional blocks in order to reach a circuit or component withinanother block from that block's periphery, the data-bus signals can beprovided directly to that circuit or component on the first die fromdata-bus interconnect on a shared interconnect layer of the second die.The data signal can be provided to its destination very quickly (e.g.,within 1 or 2 clock cycles) as it does not need to be routed from thedestination block's periphery, but rather is provided by a shortinterconnect from the shared interconnect layer above. Shorterconnections for power, clock and/or data-bus signals reduce thecapacitive load on the connections that carry these signals, which, inturn, reduces the signal skew on these lines and allows the 3D circuitto use no or less signal isolation schemes.

Stacking the IC dies also allows the dies to share power, clock and/ordata-bus circuits. For instance, in some embodiments in which the firstdie shares power, clock and/or data-bus interconnects of the second die,the first die also relies on power, clock and/or data-bus circuits thatare defined on the second die to provide the power, clock and/ordata-bus signals. This frees up space on the first die to implementother circuits and functional blocks of the 3D circuit. The resultingsavings can be quite significant because power, clock and/or data-buscircuits can often consume a significant portion of available space. Forexample, chip input/output (I/O) circuits (e.g., SERDES I/O circuits)and memory I/O circuits (e.g., DDR memory I/O circuits) can be largerthan many other circuits on an IC.

Pushing off all or some of the power and clock circuits from the firstdie to the second die also frees up space on the first die because oftenpower and clock circuits need to be isolated from other circuits and/orsignals that can affect the operation of the power and clock circuits.Also, having system level circuits on just one die allows for betterisolation of such circuits (e.g., better isolation of voltage regulatorsand/or clock circuits).

In sum, stacking the IC dies optimizes the cost and performance of achip stack system by combining certain functionalities into commoninterconnect layers and sharing these functions with multiple die in thestack. The functionalities provided by the higher-level interconnectlayers can be shared with multiple dies in the stack. The higher-levelinterconnect layers require thicker and wider metal and coarser pitch.Removing them allows each chip to be connected with a few inner levelinterconnect layers with higher density vias to enable higherperformance and lower cost. Examples of the high-level interconnectlayers include system level circuitry layers, and RDL layers. The systemcircuits include power circuits, clock circuits, data bus circuits, ESD(electro-static discharge) circuits, test circuits, etc.

The preceding Summary is intended to serve as a brief introduction tosome embodiments of the invention. It is not meant to be an introductionor overview of all inventive subject matter disclosed in this document.The Detailed Description that follows and the Drawings that are referredto in the Detailed Description will further describe the embodimentsdescribed in the Summary as well as other embodiments. Accordingly, tounderstand all the embodiments described by this document, a full reviewof the Summary, Detailed Description, the Drawings and the Claims isneeded.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features of the invention are set forth in the appendedclaims. However, for purposes of explanation, several embodiments of theinvention are set forth in the following figures.

FIG. 1 illustrates a 3D circuit of some embodiments of the invention.

FIGS. 2-4 illustrate examples of a first die in some embodiments usingpower circuits, clock circuits, and/or data-bus circuits that are formedon a substrate of a second die.

FIG. 5 illustrates an example of the top interconnect layers of thefirst and second dies having preferred wiring directions that areorthogonal to each other.

FIGS. 6-8 illustrate examples of several techniques for ensuring thatthe preferred wiring directions of the top interconnect layers of thefirst and second dies are orthogonal to each other.

FIG. 9 presents an example that illustrates a power mesh that is formedby the top interconnect layers of two face-to-face mounted dies.

FIG. 10 presents another example for sharing a power mesh between twoface-to-face mounted dies.

FIG. 11 illustrates a shared interconnect architecture in which the toptwo interconnect layers of two face-to-face mounted dies have power,ground and clock lines that form a shared power mesh and a shared clocktree.

FIGS. 12-15 presents other examples for sharing a power mesh and a clocktree between the two face-to-face mounted dies.

FIGS. 16-18 presents examples for sharing a clock tree between the twoface-to-face mounted dies.

FIGS. 19 and 20A presents examples for sharing a data bus between thetwo face-to-face mounted dies.

FIG. 20B illustrates another example of two face-to-face mounted IC diesthat form a 3D chip and that share data I/O circuits.

FIG. 21 illustrates a device that uses a 3D IC.

FIG. 22 provides an example of a 3D chip that is formed by twoface-to-face mounted IC dies that are mounted on a ball grid array.

FIG. 23 illustrates a manufacturing process that some embodiments use toproduce the 3D chip.

FIGS. 24-27 show two wafers at different stages of the fabricationprocess of FIG. 23 .

FIG. 28 illustrates an example of a 3D chip with three stacked IC dies.

FIG. 29 illustrates an example of a 3D chip with four stacked IC dies.

FIG. 30 illustrates a 3D chip that is formed by face-to-face mountingthree smaller dies on a larger die.

DETAILED DESCRIPTION

In the following detailed description of the invention, numerousdetails, examples, and embodiments of the invention are set forth anddescribed. However, it will be clear and apparent to one skilled in theart that the invention is not limited to the embodiments set forth andthat the invention may be practiced without some of the specific detailsand examples discussed.

Some embodiments of the invention provide a three-dimensional (3D)circuit that is formed by vertically stacking two or more integratedcircuit (IC) dies to at least partially overlap and to share one or moreinterconnect layers that distribute power, clock and/or data-bussignals. The shared interconnect layers include interconnect segments(also referred to as interconnect lines or wires) that carry power,clock and/or data-bus signals. In some embodiments, the sharedinterconnect layers are higher level interconnect layers.

FIG. 1 illustrates a 3D circuit 100 of some embodiments of theinvention. As shown, the circuit 100 includes two IC dies 105 and 110that are in a vertically stacked, face-to-face arrangement. Although notshown in FIG. 1 , the stacked first and second dies in some embodimentsare encapsulated into one integrated circuit package by an encapsulatingepoxy and/or a chip case. The first die 105 includes a firstsemiconductor substrate 120 and a first set of interconnect layers 125defined above the first semiconductor substrate 120. Similarly, thesecond IC die 110 includes a second semiconductor substrate 130 and asecond set of interconnect layers 135 defined above the secondsemiconductor substrate 130. In some embodiments, a subset 140 of one ormore interconnect layers of the second set interconnect layers 135 ofthe second die has interconnect wiring that carries power, clock and/ordata-bus signals that are supplied to the first IC die 105 (e.g., to theinterconnect layers and/or substrate of the first die 105). This subset140 is referred to below as the shared interconnect-layer subset.

In some embodiments, numerous electronic components (e.g., activecomponents, like transistors and diodes, or passive components, likeresistors and capacitors) are defined on the first semiconductorsubstrate 120 and on the second semiconductor substrate 130. Theelectronic components on the first substrate 120 are connected to eachother through interconnect wiring on the first set of interconnectlayers 125 to form numerous microcircuits (e.g., Boolean gates) and/orlarger circuits (e.g., functional blocks). Similarly, the electroniccomponents on the second substrate 130 are connected to each otherthrough interconnect wiring on the second set of interconnect layers 135to form additional microcircuits (e.g., Boolean gates) and/or largercircuits (e.g., functional blocks).

In some embodiments, the electronic components on one die's substrate(e.g., the first substrate 120 of the first die 105) are also connectedto other electronic components on the same substrate (e.g., substrate120) through interconnect wiring on the other die's set of interconnectlayers (e.g., the second set of interconnect layers 135 of the seconddie 110) to form additional microcircuits and/or larger circuits.

In some embodiments, power, clock and/or data-bus signals from theshared interconnect-layer subset 140 of the second die 110 are suppliedto several electronic components, microcircuits, and larger circuits ofthe first die 105. Also, in some of these embodiments, the power, clockand/or data-bus signals from the shared interconnect-layer subset 140are also supplied to electronic components, microcircuits, and largercircuits that are formed on the second substrate of the second die 110.

To form the 3D circuit 100 of FIG. 1 , the first and second dies areface-to-face stacked so that the first and second set of interconnectlayers 125 and 135 are facing each other. The top interconnect layers160 and 165 are bonded to each other through a direct bonding processthat establishes direct-contact metal-to-metal bonding, oxide bonding,or fusion bonding between these two sets of interconnect layers. Anexample of such bonding is copper-to-copper (Cu—Cu) metallic bondingbetween two copper conductors in direct contact. In some embodiments,the direct bonding is provided by a hybrid bonding technique such asDBI® (direct bond interconnect) technology, and other metal bondingtechniques (such as those offered by Invensas Bonding Technologies,Inc., an Xperi Corporation company, San Jose, Calif.). In someembodiments, DBI connects span across silicon oxide and silicon nitridesurfaces.

The DBI process is further described in U.S. Pat. Nos. 6,962,835 and7,485,968, both of which are incorporated herein by reference. Thisprocess is also described in U.S. patent application Ser. No.15/725,030, which is also incorporated herein by reference. As describedin U.S. patent application Ser. No. 15/725,030, the direct bondedconnections between two face-to-face mounted IC dies are nativeinterconnects that allow signals to span two different dies with nostandard interfaces and no input/output protocols at the cross-dieboundaries. In other words, the direct bonded interconnects allow nativesignals from one die to pass directly to the other die with nomodification of the native signal or negligible modification of thenative signal, thereby forgoing standard interfacing andconsortium-imposed input/output protocols.

In this manner, the direct bonded interconnects allow circuits to beformed across and/or to be accessed through the cross-die boundary oftwo face-to-face mounted dies. Examples of such circuits are furtherdescribed in U.S. patent application Ser. No. 15/725,030. Theincorporated U.S. Pat. Nos. 6,962,835, 7,485,968, and U.S. patentapplication Ser. No. 15/725,030 also describe fabrication techniques formanufacturing two face-to-face mounted dies.

As shown in FIG. 1 , the direct bonding techniques of some embodimentsallow a large number of direct connections 150 to be established betweenthe top interconnect layer 165 of the second die 110 and topinterconnect layer 160 of the first die 105. For these signals totraverse to other interconnect layers of the first die 105 or to thesubstrate 120 of the first die 105, the first die in some embodimentsuses other IC structures (e.g., vias) to carry these signals from itstop interconnect layer to these other layers and/or substrate. In someembodiments, more than 1,000 connections/mm², 10,000 connections/mm²,100,000 connections/mm², 1,000,000 connections/mm² or less, etc. can beestablished between the top interconnect layers 160 and 165 of the firstand second dies 105 and 110 in order to allow power, clock and/ordata-bus signals to traverse between the first and second IC dies.

The direct-bonded connections 150 between the first and second dies arevery short in length. For instance, based on current manufacturingtechnologies, the direct-bonded connections can range from a fraction ofa micron to a single-digit or low double-digit microns (e.g., 2-10microns). As further described below, the short length of theseconnections allows the signals traversing through these connections toreach their destinations quickly while experiencing no or minimalcapacitive load from nearby planar wiring and nearby direct-bondedvertical connections. The planar wiring connections are referred to asx-y wiring or connections, as such wiring stays mostly within a planedefine by an x-y axis of the 3D circuit. On the other hand, verticalconnections between two dies or between two interconnect layers arereferred to as z-axis wiring or connections, as such wiring mostlytraverses in the z-axis of the 3D circuit. The use of “vertical” inexpressing a z-axis connection should not be confused with horizontal orvertical preferred direction planar wiring that traverse an individualinterconnect layer, as further described below.

In some embodiments, the pitch between two neighboring direct-bondedconnections 150 can be extremely small, e.g., the pitch for twoneighboring connections is between 0.2 μm to 15 μm. This close proximityallows for the large number and high density of such connections betweenthe top interconnect layers 160 and 165 of the first and second dies 105and 110. Moreover, the close proximity of these connections does notintroduce much capacitive load between two neighboring z-axisconnections because of their short length and small interconnect padsize. For instance, in some embodiments, the direct bonded connectionsare less then 1 or 2 μm in length (e.g., 0.1 to 0.5 μm in length), andfacilitate short z-axis connections (e.g., 1 to 10 μm in length) betweentwo different locations on the two dies even after accounting for thelength of vias on each of the dies. In sum, the direct verticalconnections between two dies offer short, fast paths between differentlocations on these dies.

Stacking IC dies to share power, clock and/or data-bus signals betweentwo dies reduces the overall number of interconnect layers of the twodies because it allows the two dies to share some of the higher-levelinterconnect layers in order to distribute power, clock and/or data-bussignals. For example, as further described below, this sharing ofinterconnect layers allows the two dies to share one power mesh betweenthem. In some embodiments, this shared power mesh is formed by directbonding a power/ground top interconnect layer of one die (e.g., layer160 of the first die 105) with a power/ground top interconnect layer ofthe other die (e.g., layer 165 of the second die 110). In otherembodiments, this shared power mesh is formed by two interconnect layersof one die (e.g., the top two interconnect layers of the second die 110)that are shared with the other die (e.g., the first die 105). Reducingthe higher-level interconnect layers is beneficial as the wiring onthese layers often consume more space due to their thicker, wider andcoarser arrangements. In addition, the ability to share the use of theseinterconnect layers on multiple dies may reduce the congestion and routelimitations that may be more constrained on one die than another.

Stacking the IC dies in many cases also allows the wiring for deliveringthe power, clock and/or data-bus signals to be much shorter, as thestacking provides more candidate locations for shorter connectionsbetween power, clock and/or data-bus signal interconnects and thecircuit components that are to receive these signals. For instance, asfurther described below, some embodiments provide data-bus signals tocircuits on the first data through short direct-bonded connections froma data bus on a shared interconnect layer of the second die. Thesedirect-bonded connections are much shorter than connections that wouldroute data-bus signals on the first die about several functional blocksin order to reach a circuit within another block from that block'speriphery. The data signals that traverse the short direct-bondedconnections reach their destination circuits on the first die veryquickly (e.g., within 1 or 2 clock cycles) as they do not need to berouted from the periphery of the destination block. On a less-congestedshared interconnect layer, a data-bus line can be positioned over ornear a destination circuit on the first die to ensure that the data-bussignal on this line can be provide to the destination circuit through ashort direct-bonded connection.

Stacking the IC dies also allows the dies to share power, clock and/ordata-bus circuits. For instance, as shown in FIGS. 2-4 , the first die105 in some embodiments uses power circuits, clock circuits, and/ordata-bus circuits that are formed on the substrate 130 of the second die110. In these figures, the examples of power, clock and data-buscircuits are respectively voltage regulators 205, clock drivers 305, andPHY (physical layer) interfaces 405 (e.g., chip I/O interface, memoryI/O interface, etc.).

Having the first die share power, clock and/or data-bus circuits definedon the second die frees up space on the first die to implement othercircuits and functional blocks of the 3D circuit. The resulting savingscan be quite significant because power, clock and/or data-bus circuitscan consume a significant portion of available space. For example, chipI/O circuits (e.g., SERDES I/O circuits) and memory I/O circuits (e.g.,DDR memory I/O circuits) can be larger than many other circuits on anIC. Pushing off all or some of the power and clock circuits from thefirst die to the second die further frees up space on the first diebecause power and clock circuits often need to be isolated from othercircuits and/or signals that can affect the operation of the power andclock circuits. Having system level circuits on just one die also allowsfor better isolation of such circuits (e.g., better isolation of voltageregulators and/or clock circuits).

In sum, stacking the IC dies optimizes the cost and performance of achip stack system by combining certain functionalities into commoninterconnect layers and sharing these functions with multiple die in thestack. The functionalities provided by the higher-level interconnectlayers can be shared with multiple dies in the stack. The higher-levelinterconnect layers require thicker and wider metal and coarser pitch.Removing them allows each chip to be connected with a few inner levelinterconnect layers with higher density vias to enable higherperformance and lower cost. Examples of the high-level interconnectlayers include system level circuitry layers, and RDL layers. The systemcircuits include power circuits, clock circuits, data bus circuits, ESD(electro-static discharge) circuits, test circuits, etc.

Each interconnect layer of an IC die typically has a preferred wiringdirection (also called routing direction). Also, in some embodiments,the preferred wiring directions of successive interconnect layers of anIC die are orthogonal to each other. For example, the preferred wiringdirections of an IC die typically alternate between horizontal andvertical preferred wiring directions, although several wiringarchitectures have been introduced that employ 45 degree and 60 degreeoffset between the preferred wiring directions of successiveinterconnect layers. Alternating the wiring directions betweensuccessive interconnect layers of an IC die has several advantages, suchas providing better signal routing and avoiding capacitive couplingbetween long parallel segments on adjacent interconnect layers.

When face-to-face mounting of first and second IC dies, some embodimentshave the preferred wiring directions of the top interconnect layers ofthe first and second dies be orthogonal to each other in order torealize these same benefits as well as other unique benefits oforthogonal preferred wiring directions at the juncture of theface-to-face mounting. FIG. 5 illustrates an example of the topinterconnect layers of the first and second dies 505 and 510 havingpreferred wiring directions that are orthogonal to each other. In thisexample, the top interconnect layer 502 of the first die 505 has apreferred horizontal direction, while the top interconnect layer 504 ofthe second die 510 has a preferred vertical direction. As shown, thefirst die's top layer 502 can have short vertical wire segments, and thesecond die's top layer 504 can have short horizontal wire segments.However, the majority of the segments on the top layers 502 and 504 arerespectively horizontal and vertical.

Different embodiments employ different techniques to ensure that thepreferred wiring directions of the top interconnect layers of the firstand second dies are orthogonal to each other. FIGS. 6-8 illustrateexamples of several such techniques. FIG. 6 illustrates that the twodies 605 and 610 are manufactured with different processes in someembodiments. The process for the first die 605 defines the firstinterconnect layer of the first die to have a horizontal preferredwiring direction, while the process for the second die 610 defines thefirst interconnect layer of the second dies to have a vertical preferredwiring direction. As both these processes define seven interconnectlayers above the IC substrate and alternate the preferred wiringdirections between successive layers, the seventh layer of the first diehas a horizontal preferred direction while the seventh layer of thesecond die has a vertical preferred direction.

FIG. 7 illustrates an example in which the first and second dies havedifferent preferred wiring directions for their top interconnect layersbecause they have different number of interconnect layers. In thisexample, the preferred wiring direction of the first interconnect layerof both dies 705 and 710 has the same wiring direction (the horizontalin this example). However, the first die has seven interconnect layerswhile the second die has six interconnect layers. Hence, the topinterconnect layer (the seventh layer) of the first die has a horizontalpreferred wiring direction, while the top interconnect layer (the sixthlayer) of the second die has a vertical preferred wiring direction.

FIG. 8 presents an example that illustrates achieving orthogonalpreferred wiring directions between the top interconnect layers of thetwo face-to-face mounted dies 805 and 810 by rotating one of the twodies by 90 degrees. In this example, the preferred wiring directions ofthe interconnect layers of the first and second dies 805 and 810 areidentical, i.e., they both start with a horizontal preferred wiringdirection, alternate the preferred wiring directions for successivelayers, and end with a vertical preferred wiring direction.

Also, in some embodiments, the first and second dies 805 and 810 arefabricated with several masks that are jointly defined as these two diesimplement one IC design. The jointly defined masks for the two dies 805and 810 share one or more common masks in some embodiments. In otherembodiments, the first and second dies 805 and 810 are from differentmanufacturing processes and/or different foundries.

However, before face-to-face stacking the two dies 805 and 810, thesecond die is rotated by 90 degrees. This rotation in effect flips thepreferred wiring direction of each interconnect layer of the second dieto be orthogonal to the preferred wiring direction of the correspondinginterconnect layer of the first die. Thus, the top layer of the rotatedsecond die has effectively a vertical preferred wiring directioncompared to the horizontal preferred wiring direction of the top layerof the first die.

In FIG. 8 , the effective preferred wiring directions of the second dieare specified by placing these directions in quotes to indicate thatthese directions are not indicative of the manufactured preferreddirections but are indicative of the wiring directions compared to thefirst die's wiring direction and are achieved by rotating the second diewith respect to the first die. In some embodiments, the two dies 805 and810 are produced from the same mono crystalline silicon wafer or areproduced from two mono crystalline silicon wafers with the samecrystalline direction. In some of these embodiments, the two dies 805and 810 have orthogonal crystalline directions after they have beenface-to-face mounted.

Having the preferred wiring direction of the top interconnect layers ofthe first and second dies be orthogonal to each other has severaladvantages. It provides better signal routing between the IC dies andavoids capacitive coupling between long parallel segments on adjacentinterconnect layers of the two dies. Also, it allows the first andsecond dies to share the power lines on their top orthogonal layers, andthereby eliminating one or more of their power layers. Orthogonal wiringdirections on the top layers of the first and second dies increases theoverlap between the power wiring on these layers. This overlap increasesthe number of candidate locations for bonding different pairs of powerwires on the top interconnect layers of the different dies to providepower signals from one die to another die.

FIG. 9 presents an example that illustrates a power mesh 950 that isformed by the top interconnect layers 902 and 904 of the first andsecond dies 905 and 910 in some embodiments. This mesh supplies powerand ground signals to circuits defined on the first and secondsubstrates 920 and 930 of the first and second dies 905 and 910. Asshown, the top interconnect layer 902 of the first die 905 has a set ofalternating power lines 915 and ground lines 920 that traverse along thehorizontal direction, while the top interconnect layer 904 of the seconddie 910 has a set of alternating power lines 925 and ground lines 930that traverse along the vertical direction.

In some embodiments, the power/ground lines on one die's interconnectlayer are directly bonded (e.g., through DBI interconnects) tocorresponding power/ground lines on the other die's interconnect layerat each or some of the overlaps 955 between corresponding pairs of powerlines and pairs of ground lines. This direct bonding creates a veryrobust power mesh 950 for the first and second dies without using twodifferent interconnect layers for each of these two dies. This frees upat least one interconnect layer on each die and in total eliminates twointerconnect layers from the 3D circuit (formed by the face-to-facebonded dies 905 and 910) by having the two dies share one power mesh.Also, the face-to-face mounted top interconnect layers allow thicker andwider interconnect lines to be used for the power signals, which, inturn, allows these signals to face less resistance and suffer lesssignal degradation.

In some embodiments, the power and ground signals are supplied by powercircuitry defined on the substrate of the second die 910 as describedabove by reference to FIG. 2 . In some of these embodiments, the powerand ground signals from the power circuitry are supplied from the seconddie's substrate through vias to the power and ground lines on the topinterconnect layer 904 of the second die 910. From this interconnectlayer 904, these signals are supplied through direct bonded connections(e.g., DBI connections) to power and ground lines on the topinterconnect layer 902 of the first die 905, from where they aresupplied to circuits and other interconnect layers of the first die 905.

FIG. 10 presents another example for sharing a power mesh 1050 betweenthe first and second dies 1005 and 1010 in some embodiments. In thisexample, the power mesh 1050 is formed by the top two interconnectlayers 1002 and 1004 of the second die 1010. Other than both of theseinterconnect layers belonging to the second die 1010, these twointerconnect layers 1002 and 1004 are similar to the interconnect layers902 and 904. Specifically, the interconnect layer 1002 has alternatingpower lines 1015 and ground lines 1020 while the interconnect layer 1004has alternating power lines 1025 and ground lines 1030, with viasdefined at each or some of the overlaps 1055 between corresponding pairsof power lines and pairs of ground lines.

The power mesh architecture of FIG. 10 consumes two interconnect layersof the second die 1010 but does not use any interconnect layers of thefirst die. Hence, like the power mesh 950, the power mesh 1050eliminates in total two interconnect layers from the 3D circuit byhaving the two dies share one power mesh. Also, defining the power meshwith the top two interconnect layers of the die 1010 allows thicker andwider interconnect lines to be used for the power signals, which, inturn, allows these signals to face less resistance and suffer lesssignal degradation.

In some embodiments, the power and ground signals are supplied by powercircuitry defined on the substrate of the second die 1010 to the powerand ground lines 1015-1030 on the top-two interconnect layers 1002 and1004 of the second die 1010. From these interconnect layers 1002 and1004, these signals are supplied to power and ground interconnect linesand/or pads on the top interconnect layer of the first die 1005 throughdirect-bonded connections (e.g., DBI connections) between the first andsecond dies 1005 and 1010. From the top interconnect layer of the firstdie 1005, the power and ground signals are then supplied through vias toother interconnect layers of the first die 1005 and to circuits definedon the substrate of the first die.

In power mesh architectures of FIGS. 9 and 10 , as well as some of theother figures described below, the direct connections or vias thatestablish the electrical connections between two power lines on twodifferent layers, or two ground lines on two different layers,electrically shield signals that traverse vertically in between theseconnections/vias through their own vertical connections or vias thattraverse different interconnect layers on the same die or differentdies. Also, in these examples, the power lines distribute power andground signals. One of ordinary skill will realize that in otherembodiments, the shared power distribution networks between two or morevertically stacked dies distribute other types of power signals, such asreference voltages (V_(REF)) and low power state voltages.

Also, in some embodiments, a first power mesh is defined on the top twointerconnect layers of a first die, while a second power mesh is definedon the top two interconnect layers of a second die that is face to facemounted with the first die through a direct bonding process. In some ofthese embodiments, the direction of the power/ground interconnects onthe top interconnect layer of the first die is orthogonal to thedirection of the power/ground interconnects on the top interconnectlayer of the second die.

In other embodiments, two dies that are face-to-face mounted through adirect bonding process (e.g., a DBI process) have power/ground lines onthe top two interconnect layers of a first die (like layers 1002 and1004 of FIG. 10 ), but power/ground lines only on the top interconnectlayer of the second die. In some of these embodiments, the direction ofthe power/ground interconnects on the top interconnect layer of thefirst die is orthogonal to the direction of the power/groundinterconnects on the top interconnect layer of the second die. In thisface-to-face mounted 3D chip arrangement, one power sub-mesh is formedby the top two interconnect layers of the first die, while another powersub-mesh is formed by the top interconnect layers of the first andsecond dies. These two sub-meshes form a three-layer shared power meshon the two dies.

The shared power meshes that are formed by the top interconnect layersof one or both dies are used in some embodiments to shield other typesof interconnect lines on these layers or between these layers.Specifically, some embodiments not only share a power mesh between twoface-to-face mounted dies, but also share a clock tree that is formed onone or two interconnect layers that are shared between the two dies. Insome embodiments, the clock tree is formed on the same sharedinterconnect layers that form the power mesh, while in other embodimentsthe interconnect layer or layers that contain the clock mesh are inbetween the interconnect layers that form the power mesh. The power meshin some embodiments shields the clock lines from capacitive coupling ofthe other clock and data interconnect lines.

FIG. 11 illustrates a shared interconnect architecture of someembodiments. In this architecture, the top two interconnect layers 1115and 1120 of two face-to-face mounted dies 1105 and 1110 (that form a 3Dstacked chip 1100) have power, ground and clock lines that form a powermesh 1150 and a clock tree 1160. FIG. 11 has four sets of schematics.The first set shows the two face-to-face mounted dies 1105 and 1110. Thesecond set shows dies 1105 and 1110, and expanded views of the top twointerconnect layers 1115 and 1120 of these two dies. The top half of thethird set of schematics shows just the power and ground lines on the toptwo interconnect layers 1115 and 1120, while the bottom half of thethird set shows just the clock lines on these two layers. Lastly, tophalf of the fourth set of schematics shows the power mesh formed by thepower and ground lines of the top two interconnect layers 1115 and 1120,while the bottom half of this set shows the clock tree 1160 formed bythe clock lines on these two layers.

As shown in the second and third sets of schematics of FIG. 11 , the topinterconnect layer 1115 of the first die 1105 includes horizontal powerlines 1130, ground lines 1135 and clock lines 1140, while the topinterconnect layer 1120 of the second die 1110 includes vertical powerlines 1130, ground lines 1135 and clock lines 1140. In these schematics,the power/ground lines 1130 and 1135 are thinner, long solid lines,while the clock lines 1140 are thicker, shorter line segments.

The power and ground lines 1130 and 1135 on each interconnect layeralternate in their order (i.e., a power line is followed by a groundline, which is followed by a power line, and so on). Also, one set ofclock line segments are placed between each neighboring pair of powerand ground lines 1130 and 1135. Thus, each clock line segment 1140 oneach interconnect layer is between two power/ground lines 1130 and 1135that shield the clock line segment from nearby clock and data lines andthereby reduce the capacitive coupling between the clock line segmentand the nearby clock and data lines. Also, by virtue of being in the topinterconnect layers, the clock line segments are thicker and wider,which, in turn, reduces their resistance and allows the clock signalsthat they carry to be driven longer distances.

The horizontal and vertical clock line segments on the interconnectlayers 1115 and 1120 form a shared H-tree clock structure 1160 thatdistributes a clock signal to the circuits on the first and second dies1105 and 1110. The H-tree clock structure will be further describedbelow. To form the clock tree 1160, each horizontal clock line segmenton the interconnect layer 1115 is connected through at least one directbonded connection (e.g., DBI connection) to at least one vertical clockline segment on the interconnect layer 1120. Some of clock line segmentson one top interconnect layer (e.g., layer 1115) connect to three clockline segments on the other interconnect layer (e.g., layer 1120) throughthree direct bonded connections (e.g., DBI connections). Similarly, toform the power mesh 1150, (1) each power line on one interconnect layer(e.g., layer 1115) connects through one or more direct bondedconnections (e.g., DBI connections) to one or more power lines on theother interconnect layer (e.g., layer 1120), and (2) each ground line onone interconnect layer (e.g., layer 1115) connects through one or moredirect bonded connections (e.g., DBI connections) to one or more groundlines on the other interconnect layer (e.g., layer 1120).

The power mesh 1150 and clock tree 1160 eliminate two or moreinterconnect layers from the 3D circuit by having the two dies share twointerconnect layers 1105 and 1110 that together form the power mesh 1150and the clock tree 1160. On each die 1105 or 1110, the power, ground andclock signals are distributed among the interconnect layers of that diethrough vias between the interconnect layers. In some embodiments, powerand clock circuits are defined on the substrate of only one of the dies(e.g., on the substrate of the second die 1110). In other embodiments,the power circuits are defined on the substrate of one die (e.g., thesubstrate of the first die 1105), while the clock circuits are definedon the substrate of the other die (e.g., the substrate of the second die1110). In other embodiments, power and/or clock circuits are defined onthe substrate of both dies 1105 and 1110.

The H-tree clock structure includes a hierarchical series ofH-structures, with each H-structure distributing the same clock signalfrom the center of the H-structure to the outer four corners of theH-structure, where the signal is passed to the center of another,smaller H-structure, until the clock signal reaches the outer corner ofthe smallest H-structures. The center of the largest H-structurereceives the clock signal from a clock circuit that is defined on thesecond die's substrate in some embodiments. In other embodiments, thissignal is supplied to other locations of the H-structure from the clockcircuit on the second die's substrate, or to a location on theH-structure from a clock circuit on the first die's substrate. In someembodiments, the clock signal is distributed from H-tree structure 1160to circuits and interconnects of the first and second dies through vias.

FIG. 12 presents another example for sharing a power mesh 1250 and aclock tree 1260 between the first and second dies 1205 and 1210 in someembodiments. In this example, the power mesh 1250 and the clock tree1260 are formed by the top two interconnect layers 1215 and 1220 of asecond die 1210 that is face-to-face mounted through direct bondedconnections with a first die 1205 to form a 3D chip 1200. Other thanboth of these interconnect layers belonging to the second die 1210,these two interconnect layers 1215 and 1220 are similar to theinterconnect layers 1115 and 1120.

Specifically, each interconnect layer 1215 or 1220 has alternating powerlines 1225 and ground lines 1230 and clock line segments betweenneighboring pairs of power and ground lines. Vias are defined at each orsome of the overlaps between corresponding pairs of power lines,corresponding pairs of ground lines and corresponding pairs of clockline segments, in order to create the power mesh 1250 and the clock tree1260. The shared interconnect architecture of FIG. 12 eliminates two ormore interconnect layers from the 3D circuit by having the two diesshare the two interconnect layers 1215 and 1220 that form the power mesh1250 and the clock tree 1260.

In some embodiments, the power, ground and clock signals are supplied bypower and clock circuitry defined on the substrate of the second die1210 to the power, ground and clock lines on the interconnect layers1215 and 1220 of the second die 1210. From these interconnect layers1215 and 1220, the power, ground and clock signals are supplied topower, ground and clock interconnect lines and/or pads on the topinterconnect layer of the first die 1205 through direct-bondedconnections (e.g., DBI connections) between the first and second dies1205 and 1210. From the top interconnect layer of the first die 1205,the power, ground and clock signals are then supplied through vias toother interconnect layers of the first die 1205 and to circuits definedon the substrate of the first die. In some embodiments, power circuitsand/or clock circuits are also defined on the substrate of the first die1205.

FIG. 13 illustrates another shared interconnect architecture of someembodiments. In this example, a power mesh 1350 and a clock tree 1360are formed by the top interconnect layer 1315 of a first die 1305 andthe top two interconnect layers 1320 and 1325 of a second die 1310,which is face-to-face mounted to the first die 1305 through directbonded connections to form a 3D chip 1300. The shared architecture ofthis example is similar to the shared interconnect architecture of FIG.9 , except that the top interconnect layer 1320 of the second die 1310contains a shared H-tree clock structure 1350 for distributing a clocksignal to the circuits on the first and second dies 1305 and 1310, andthis interconnect layer 1320 is between two power/ground interconnectlayers 1315 and 1325 of the first and second dies 1305 and 1310. Thisplacement of the H-tree clock structure between the power/groundinterconnect layers 1315 and 1325 shields the clock line segments inthis structure from capacitively coupling to interconnect lines thatcarry data and other signals on other interconnect layers of the firstand second dies 1305 and 1310.

The power/ground lines in some embodiments alternate on each of theinterconnect layers 1315 and 1325. Also, in some embodiments, thepower/ground lines on the interconnect layer 1325 of the second dieconnect to pads on this die's interconnect layer 1320, and these padsare connected through direct bonded connections (e.g., DBI connections)to power lines on the interconnect layer 1315. The power/ground signalsin some embodiments are distributed to other interconnect and substratelayers on each die through vias.

Also, in some embodiments, the clock signal is distributed from H-treestructure 1360 to circuits and interconnects of the second die throughvias, while it is distributed from this structure 1360 to circuits andinterconnects of the first die through direct-bonded connections betweenthis structure and clock pads on layer 1315 of the first die. Thedirect-bonded connections in some embodiments emanate from the cornersof some of the H-structures and travel along the z-axis. The center ofthe largest H-structure in this clock tree receives the clock signalfrom a clock circuit that is defined on the second die's substrate insome embodiments. In other embodiments, this signal is supplied to otherlocations of the H-structure from the clock circuit on the second die'ssubstrate, or to a location on the H-structure from a clock circuit onthe first die's substrate.

FIG. 14 illustrates yet another shared power/clock interconnectarchitecture of some embodiments. This architecture 1400 is similar tothe power/clock interconnect architecture 1300 of FIG. 13 , except thatthe power and clock interconnect layers 1415, 1420 and 1425 are allinterconnect layers of the second die 1410. In this example, the firstdie 1405 does not contain any interconnect layer that is dedicated toeither the power or clock lines. Also, in this example, the H-tree clockstructure 1460 is again between the power/ground interconnect layers1415 and 1425 of the second die 1410, and hence its clock line segmentsare shielded by these power/ground interconnect layers from capacitivecouplings to other interconnect lines that carry data and other signalson other interconnect layers of the first and second dies 1405 and 1410.

In the architecture 1400, the power, ground and clock signals aresupplied to circuits and interconnects of the first die by directlybonding these circuits and interconnects through direct-bondedconnections from the power/ground lines and clock lines/pads on layer1415 of the second die to lines/pads on the top layer 1412 of the firstdie 1405. The power, ground and clock signals are supplied in someembodiments to circuits, interconnects, and pads of the second diethrough vias. Similarly, in some embodiments, the power, ground andclock signals are supplied from the top layer 1412 of the first die 1405to circuits and interconnects of the first die 1405 through vias.

FIG. 15 illustrates yet another shared power/clock interconnectarchitecture of some embodiments. This architecture 1500 is similar tothe power/clock interconnect architecture 1300 of FIG. 13 . However, inthe architecture 1500, the H-tree structure 1560 is implemented by thetop interconnect layers 1515 and 1520 of two dies 1505 and 1510, whichare face-to-face mounted through direct bonded connections (e.g., DBIconnections) to form a 3D chip 1500. The clock interconnect layer 1515is the top interconnect layer of the first IC die 1505 and has thehorizontal segments of the H-tree structure 1560. The clock interconnectlayer 1510 is the top interconnect layer of the second IC die 1510, andhas the vertical segments of the H-tree structure 1560.

The vertical and horizontal segments of the H-tree structure 1560 areconnected to each other through direct-bonded connections (e.g. DBIconnections). The center of the largest H-structure receives the clocksignal from a clock circuit that is defined on the second die'ssubstrate in some embodiments. In other embodiments, this signal issupplied to other locations of the H-structure from the clock circuit onthe second die's substrate, or to a location on the H-structure from aclock circuit on the first die's substrate. In some embodiments, theclock signal is distributed from the clock lines of the interconnectlayer 1515 of the first die 1505 to circuits and interconnects of thefirst die through vias defined in the first die. Similarly, the clocksignal is distributed from the clock lines on the interconnect layer1520 of the second die 1510 to circuits and interconnects of the seconddie through vias.

As shown, the H-tree clock structure 1560 is between the interconnectlayer 1525 of the first die 1505 and the top interconnect layer 1530 ofthe second die 1510. Like the position of the H-tree structure 1360, theplacement of the H-tree clock structure 1560 between the power/groundinterconnect layers 1525 and 1530 shields the clock line segments inthis structure from capacitively coupling to interconnect lines thatcarry data and other signals on other interconnect layers of the firstand second dies 1505 and 1510.

In this example, the power/ground layers 1525 and 1530 connect topower/ground pads on clock interconnect layers 1515 and 1520 throughvias. The power/ground pads on one of these interconnect layers (e.g.,layer 1515) connect to corresponding power/ground pads on the otherinterconnect layer (e.g., layer 1520) through direct-bonded connections(e.g., DBI connections). Through these vias and direct-bondedconnections, corresponding pairs of power/ground lines are connected onthe interconnect layers 1525 and 1530 to form the power mesh 1550.

The power/ground signals in some embodiments are distributed to otherinterconnect and substrate layers on each die through vias. In someembodiments, the four power/clock interconnect layers 1515, 1520, 1525and 1530 are the interconnect layers of one of the dies (e.g., thesecond die 1510), and these four layers are shared by the first die1505. In other embodiments, three of these interconnect layers belong toone die and one of them belongs to another die.

In some embodiments, the 3D chip structure that is formed by twoface-to-face mounted dies has one or more clock interconnect layers inbetween a full power mesh that is formed on the first die and afull/half power mesh that is formed on the second die. A full power meshon a die in some embodiments includes at least two interconnect layersthat contain power/ground interconnect lines. In some of theseembodiments, a partial power mesh on a die includes one interconnectlayer that contains power/ground interconnect lines, and that connectsthrough z-axis vertical connections (e.g., via and DBI connections) tothe power mesh of the other die. In some of these embodiments, the fullor partial power mesh layer(s) on one die do not include the topinterconnect layer of that die as the top layer is used to carry clockor data interconnect lines (like the top interconnect layers 1515 and1520 of FIG. 15 , which carry clock lines).

In some embodiments, two vertically stacked IC dies do not sharepower-distributing interconnect layers but share interconnect layers forsharing clock signal or signals. FIGS. 16-18 illustrate examples of twosuch shared interconnect architectures. In FIG. 16 , two dies 1605 and1610 are face-to-face mounted through direct bonded connections to forma 3D chip 1600. The top interconnect layer 1620 of the die 1610 includesa clock tree 1660 that is used (1) to distribute a clock signal to otherinterconnect layers of the die 1610 through vias of this die, and (2) todistribute the clock signal to other interconnect layers of the die 1605through direct-bonded connections (e.g., DBI connections) to the topinterconnect layer 1615 of the die 1605 and then through vias of thisdie 1605.

As in the examples illustrated in FIGS. 13 and 14 , the clock tree 1660is an H-tree structure. One of ordinary skill will realize that otherembodiments use other types of clock distribution structures. The centerof the largest H-structure receives the clock signal from a clockcircuit that is defined on the second die's substrate in someembodiments. In some of these embodiments, the first IC die 1605 doesnot include a clock circuit that generates a clock signal. In otherembodiments, this signal is supplied to other locations of theH-structure from the clock circuit on the second die's substrate, or toa location on the H-structure from a clock circuit on the first die'ssubstrate.

FIG. 17 illustrates two dies 1705 and 1710 are face-to-face mountedthrough direct bonded connections to form a 3D chip 1700. In thisexample, the top interconnect layers 1715 and 1720 of these two dies1705 and 1710 respectively include horizontal clock line segments 1735and vertical clock line segments 1740 that together form a clock tree1760 that is used to distribute a clock signal to other interconnectlayers of the dies 1705 and 1710. The horizontal and vertical linesegments on the top interconnect layers 1715 and 1720 are connectedthrough direct-bonded connections (e.g., DBI connections) in order toform the H-tree clock structure 1760.

One or more clock line segments on the top layer 1720 of the second die1710 in some embodiments receive the clock signal from a clock circuitthat is defined on the second die's substrate. In some embodiments, theclock signal is distributed from the clock lines on the interconnectlayer 1715 of the first die 1705 to circuits and interconnects of thefirst die through vias of the first die. Similarly, the clock signal isdistributed from the clock lines on the interconnect layer 1720 of thesecond die 1710 to circuits and interconnects of the second die throughvias.

FIG. 18 illustrate yet another shared interconnect structure fordistributing clock signals between two face-to-face mounted IC dies.This architecture is similar to the architecture of FIG. 17 , exceptthat in FIG. 18 the horizontal and vertical clock interconnect layers1815 and 1820 both belong to a second die 1810 that is face-to-facemounted through direct bonded connections to a first die 1805 to form a3D chip 1800. In this architecture, vias between the interconnect layers1815 and 1820 of the second die establish the connections between theclock lines on these layers in order to create the clock structure 1860(i.e., the H-tree structure 1860) in this example.

Direct bonded connections between the first and second dies 1805 and1810 then supply the clock signal from this clock structure to clocklines/pads on the top interconnect layer of the first die 1805. Theclock signal is then passed to other interconnect and substrate layersof the first die 1805 through vias. The clock signal is also passed toother interconnect and substrate layers of the second die 1810 throughvias. In some embodiments, a clock circuit on the second die's substratesupplies the clock signal to one or more clock line segments oninterconnect layer 1815 and/or interconnect layer 1820 of the second die1810. In other embodiments, the clock signal is generated by a clockcircuit defined on the substrate of the first die 1805.

One of the unique features of the 3D chips illustrated in FIGS. 11-18 isthat in these chips, the clock lines are moved to the top interconnectlayers of a die, or next to the top interconnect layer of the die.Typically, clock lines are not in the top interconnect layers as such aplacement would expose the clock signals/lines to interfering signalsoutside of the chip. However, the face-to-face mounted dies of FIGS.11-18 can place the clock lines in their top interconnect layers asthese layers are very well isolated from signals outside of their 3Dchips because these interconnect layers are effectively in the middle ofthe die stack.

In addition to isolating the clock signals, the face-to-face mounted topinterconnect layers allow thicker and wider interconnect lines to beused for the clock signals. These signals have less resistance andsuffer less signal degradation. Hence, the clock signals can be drivenlonger distance with no clock signal regeneration (which would requirethe clock signals to travel to the buffer circuits formed on asemiconductor substrate) or with less clock signal regeneration. Thislower resistance advantage (i.e., less signal degradation advantage) ofthicker and wider interconnects on upper interconnect layers is alsoenjoyed by power and data interconnect line segments that are defined onthe upper interconnect layers and that are shared between two or morevertically stacked IC dies (e.g., two face-to-face mounted IC dies).

As mentioned above, stacking IC dies also allows two or more dies toshare a data bus on one or more share interconnect layers. FIG. 19illustrates an example of one such shared interconnect layerarchitecture that allows two face-to-face mounted IC dies to share adata bus and a data storage that are defined on one of the dies. In thisexample, the data storage is an on-chip cache 1960. In otherembodiments, the shared data storage is any other type of storage. InFIG. 19 , the two face-to-face mounted IC dies 1905 and 1910 (that forma 3D chip 1900) share a data bus 1950 that is defined on a topinterconnect layer 1920 of the second die 1910. As shown, this topinterconnect layer 1920 connects to the top interconnect layer 1915 ofthe first die 1905 through direct bonded connections (e.g., DBIconnections).

Although a data bus does not necessarily need to include parallelinterconnect lines, the data bus 1950 in this example includes severalparallel interconnect lines that connect to other interconnect lines onthe first and second dies through vias and direct-bonded connections atone or more locations along each interconnect line. These lines areshown to be physically parallel, but in other cases, they are justtopologically parallel (e.g., on one end, they connect to severaladjacent locations at one position of a die or interconnect layer, whileon another end, they connect to several other adjacent locations atanother position in a die or interconnect layer). The data bus 1950connects through interconnect lines and vias to an input/outputinterface 1955 of a cache storage 1960 that is defined on the substrate1965 of the second die 1910. Through interconnect lines and vias, thedata bus 1950 also connects to circuits defined on the second die 1910,so that through these connections and the I/O interface 1955 thesecircuits can receive output data read from the cache storage 1960, andprovide input data for storing in the cache storage 1960.

Through the direct bonded connections, the data bus 1950 also connectsto interconnect lines/pads on the top interconnect layer 1915 of thefirst die 1905. These interconnect lines/pads on the interconnect layer1915 connect to circuits on the first die 1905 through interconnectlines and vias of the first die 1905. Through these connections (i.e.,the interconnect lines, vias and direct-bonded connections) and the I/Ointerface 1955, the circuits defined on the first die 1905 can receiveoutput data read from the cache storage 1960, and provide input data forstoring in the cache storage 1960.

Stacking the IC dies so that they can share one or more data busesallows the wiring for delivering the data to be much shorter, as thestacking provides more candidate locations for shorter connectionsbetween data bus interconnects and the circuit components that are toreceive these signals. For instance, instead of routing data signals onthe second die about several functional blocks in order to reach acircuit or component within another block from that block's periphery,the data signals can be provided directly to that circuit or componenton the second die from data-bus interconnect on the shared interconnectlayer of the first die. The data signal can be provided to itsdestination very quickly (e.g., within 1 or 2 clock cycles) as it doesnot need to be routed from the destination block's periphery, but ratheris provided by a short interconnect from the shared interconnect layerabove. Shorter connections for data signals reduce the capacitive loadon the connections that carry these signals, which, in turn, reduces thesignal skew on these lines and allows the 3D circuit to use no or lesssignal isolation schemes.

FIG. 20A illustrates another example of two face-to-face mounted IC diessharing resources. In this example, the circuits of first and seconddies 2005 and 2010 of the two dies share data I/O circuitry, whichincludes an I/O interface 2025, an external data I/O unit 2030 (e.g.,level shifting drivers), and a data I/O bus 2022 formed by a number ofdata bus lines. The data I/O unit 2030 can be an external memory I/Ounit or another data interface unit, such as a SerDes unit. In FIG. 20A,the two face-to-face mounted IC dies 2005 and 2010 form a 3D chip 2000.Through silicon vias (TSVs) are defined on the backside of the seconddie 2010. Through these TSVs and the I/O interface, data is received andsupplied to the data I/O unit 2030 defined on the substrate of thesecond die 2010.

The data I/O unit 2030 connects through interconnect lines and vias ofthe second die to the data bus 2022 that is defined on a topinterconnect layer 2020 of the second die 2010. As shown, this topinterconnect layer 2020 connects to the top interconnect layer 2015 ofthe first die 2005 through direct bonded connections (e.g., DBIconnections). In this example, the data bus 2022 is again shown to haveseveral parallel interconnect lines that connect to other interconnectlines on the first and second dies through vias and direct-bondedconnections at one or more locations along each interconnect line.However, as mentioned above, the interconnect lines of a data bus do notnecessarily need to be parallel.

Through interconnect lines and vias, the data bus 2022 connects tocircuits defined on the second die 2010, so that through theseconnections these circuits can receive data from and supply data to thedata I/O unit 2030. Through the direct bonded connections, the data bus2022 also connects to interconnect lines/pads on the top interconnectlayer 2015 of the first die 2005. These interconnect lines/pads on theinterconnect layer 2015 connect to circuits on the first die 2005through interconnect lines and vias of the first die 2005. Through theseconnections (i.e., the interconnect lines, vias and direct-bondedconnections), the circuits defined on the first die 2005 can receivedata from and supply data to the data I/O unit 2030.

Some embodiments distribute an IO circuit between two or more verticallystacked IC dies. For instance, some embodiments distribute a SerDescircuit between two vertically stacked IC dies. A SerDes circuitincludes digital (logic) portions and analog portions. In someembodiments, the digital portions of the SerDes circuit are implementedon a first IC die, while the analog portions are implemented on a secondIC die that is face-to-face mounted or otherwise vertically stacked withthe first die. This IO interface has to involve the interaction betweenthese two layers before signals are passed to the core circuitry. Onlythe two layers together complete the IO circuitry.

FIG. 20B illustrates another example of two face-to-face mounted IC diesthat form a 3D chip 2052 and that share data I/O circuits. In thisexample, the I/O circuitry is defined on both dies 2055 and 2060 inorder to reduce the area that the I/O circuitry consumes on each die.The I/O circuitry in this example includes two sets of power and groundrails 2062-2068, ESD (electro-static discharge) circuits 2073, drivers2074, and decoupling capacitors (not shown).

The power/ground rails include two power rails 2062 and 2066 on the topinterconnect layer 2070 of the second die 2060 and two ground rails 2064and 2068 on the top interconnect layer 2072 of the first die 2055. Thepower and ground rails 2062 and 2064 are the I/O interface power andground rails that carry the power and ground signals for the I/Ocircuitry (e.g., I/O drivers). The power and ground rails 2066 and 2068are the core power and ground rails that carry the power and groundsignals for the core circuits of the first and second dies. The corecircuits of the dies are the circuits that perform the computationoperations of the dies.

In some embodiments, each power or ground rail is a rectangular ringformed by four rectangular segments, with each segment spanning one ofthe four sides of the die and connecting to two other rectangularsegments of the same rail. In other embodiments, each power rail is nota contiguous ring that spans the entire periphery of the die, as it hasone or more discontinuities (e.g., at the corners of the interconnectlayer on which it resides). Also, while showing power and ground railson the top interconnect layers 2070 and 2072, one of ordinary skill willrealize that in some embodiments power and ground rails exist on otherinterconnect layers of the dies (e.g., power rails on severalinterconnect layers of one die, and ground rails on several interconnectlayers of the other die).

Multiple drivers 2074 are formed on the substrate 2082 of the first die2055. When signals traverse from circuits outside of the dies to corecircuits of the die, the drivers 2074 level shift these signals toconvert them from their external voltage levels to internal voltagelevels. Similarly, when signals traverse from the core circuits of thedie to circuits outside of the dies, the drivers 2074 level shift thesesignals to convert them from their internal voltage levels to externalvoltage levels. The drivers 2074 also provide signal buffering. Toperform their operations (e.g., level shifting operations), the driversreceive power and ground signals from the power and ground rails2062-2068.

In some embodiments, the substrate 2080 of the second die 2060 includesthe signal pads that are connected through TSVs to signal pads on thebackside of the second die 2060. These backside signal pads areconnected to external interconnects (e.g., micro bump arrays) thatreceive signals from and supply signals to external circuits outside ofthe 3D chip 2052. Through these backside signal pads, the signals padson the front side of the second die substrate 2080 receive signals fromexternal circuits for the I/O circuitry, and supply signals from the I/Ocircuitry to external circuits. One of ordinary skill will realize thatother embodiments use other structures (e.g., copper pillars connectedthrough interposers) to supply signals to the dies.

As shown, the second die 2060 includes the ESD circuits 2073 that aredefined on its substrate, while the first die 2055 includes the drivers2074 that are defined on its substrate. The ESD circuits are formaintaining signal stability inside of the chip. The ESD circuits aredesigned in some embodiments to dissipate quickly external irregularsignal surges, in order to maintain signal stability inside of the chip.Each die 2055 or 2060 also includes decoupling capacitors that are formaintaining signal stability inside of the chip by eliminating signalnoise from affecting signal quality on the chip.

The power or ground rail (I/O or core) on the top interconnect layer ofeach die has to provide its power signal or ground signal to the otherdie through the top interconnect layer of the other die. In someembodiments, this is done by having the power signal or ground signaltraverse down one layer on the same die with one or more vias, traversealong interconnect lines on that layer, and then traverse back up alongone or more vias to one or more pads on the top interconnect layer ofits die. These pads have direct-bonded connections (e.g., DBIconnections) to pads on the top interconnect layer of the other die. Thepads on the other die then distribute to circuits on the other die thereceived power or ground signals through vias and interconnect lines.Also, between respective power and ground rails (e.g., I/O power andground rails, or core power and ground rails), some embodiments definedecoupling capacitors in the face-to-face mounted layer coupling the twodies, in order to suppress the effect of signal noise on the powersupply.

In some embodiments, the core power and ground rails 2066 and 2068respectively connect to interior power and ground lines on the sameinterconnect layers as the rails 2066 and 2068. These interior power andground lines in some embodiments form an interior power mesh, such asthe power mesh shown in either FIG. 9 or 10 . Also, in some embodiments,the top interconnect layer of each die 2055 or 2060 has additionaldirect-bonded connections with the top interconnect layer of the otherdie in order to receive inputs for the I/O circuitry components (e.g.,for the ESD circuits, drivers, etc.) from the other die, and to provideoutputs from the I/O circuitry components (e.g., for the ESD circuits,drivers, etc.) to the other die.

In prior IC designs, the power/ground rails for the I/O circuitry and ICcore are typically defined as four concentric rectangular rings that areplaced on a single die along with the decoupling capacitors, drivers,and ESD circuits of the I/O circuitry. Placing these components on onedie requires the I/O circuitry to consume a lot of area on the peripheryof an IC die. This, in turn, leads to larger dies or leaves less spacefor the IC core. The 3D chip 2052, on the other hand, does not sufferthese shortcomings as its I/O circuitry is split on the two dies 2055and 2060. Also, by placing the power and ground rails (for the I/O andthe core) on different dies, the 3D chip 2052 has far less area devotedto the power and ground rails, leaving more space to the circuits of theIC's core.

One of ordinary skill will understand that the 3D chip 2052 presentsonly one way by which the I/O circuits and power rails can bedistributed among two vertically stacked (e.g., two face-to-face mounteddies). Other embodiments use other techniques to distribute the I/Ocircuits and power rails. For instance, in other embodiments, one I/Opower rail is on the periphery of a top interconnect layer of a firstdie, while another I/O power rail is closer to the center of the topinterconnect layer(s) of a second die vertically stacked (e.g.,face-to-face mounted) with the first die. Still other embodiments definemultiple stripes of I/O rails on the higher interconnect layers of twovertically stacked dies and then define multiple cores between differentstripes. Accordingly, the architecture presented in FIG. 22B is onlyillustrative of how some embodiments distribute the I/O circuit andpower rails between two vertically stacked dies.

FIG. 21 illustrates a device 2102 that uses a 3D IC 2100 (like any ofthe 3D IC 100, 900-2000). In this example, the 3D IC 2100 is formed bytwo face-to-face mounted IC dies 2105 and 2110 that have numerous directbonded connections 2115 between them. In other examples, the 3D IC 2100includes three or more vertically stacked IC dies. As shown, the 3D ICdie 2100 includes a cap 2150 that encapsulates the dies of this IC in asecure housing 2125. On the back side of the die 2110 one or more TSVsand/or interconnect layers 2106 are defined to connect the 3D IC to aball grid array 2120 (e.g., a micro bump array) that allows this to bemounted on a printed circuit board 2130 of the device 2102. The device2102 includes other components (not shown). In some embodiments,examples of such components include one or more memory storages (e.g.,semiconductor or disk storages), input/output interface circuit(s), oneor more processors, etc.

In some embodiments, the first and second dies 2105 and 2110 are thefirst and second dies shown in any of the FIGS. 1-20 . In some of theseembodiments, the second die 2110 receives power, clock and/or data bussignals through the ball grid array, and routes the received signals toshared power, clock and/or data bus lines on its shared interconnectlayer(s), from where the received signals can be supplied to theinterconnects/circuits of the first die through direct bondedconnections between the first and second dies 2105 and 2110.

FIG. 22 provides another example of a 3D chip 2200 that is formed by twoface-to-face mounted IC dies 2205 and 2210 that are mounted on a ballgrid array 2240. In this example, the first and second dies 2205 and2210 are face-to-face connected through direct bonded connections (e.g.,DBI connections). As shown, several TSVs 2222 are defined through thesecond die 2210. These TSVs electrically connect to interconnects/padson the backside of the second die 2210, on which multiple levels ofinterconnects are defined.

In some embodiments, the interconnects on the backside of the second die2210 create the signal paths for defining one or more system levelcircuits for the 3D chip 2200 (i.e., for the circuits of the first andsecond dies 2205 and 2210). Examples of system level circuits are powercircuits, clock circuits, data I/O signals, test circuits, etc. In someembodiments, the circuit components that are part of the system levelcircuits (e.g., the power circuits, etc.) are defined on the front sideof the second die 2210. The circuit components can include activecomponents (e.g., transistors, diodes, etc.), or passive/analogcomponents (e.g., resistors, capacitors (e.g., decoupling capacitors),inductors, filters, etc.

In some embodiments, some or all of the wiring for interconnecting thesecircuit components to form the system level circuits are defined oninterconnect layers on the backside of the second die 2210. Using thesebackside interconnect layers to implement the system level circuits ofthe 3D chip 2200 frees up one or more interconnect layers on the frontside of the second die 2210 to share other types of interconnect lineswith the first die 2205. The backside interconnect layers are also usedto define some of the circuit components (e.g., decoupling capacitors,etc.) in some embodiments. As further described below, the backside ofthe second die 2210 in some embodiments can also connect to the front orback side of a third die.

In some embodiments, one or more of the layers on the backside of thesecond die 2210 are also used to mount this die to the ball grid array2240, which allows the 3D chip 2100 to mount on a printed circuit board.In some embodiments, the system circuitry receives some or all of thesystem level signals (e.g., power signals, clock signals, data I/Osignals, test signals, etc.) through the ball grid array 2240 connectedto the backside of the third die.

In some embodiments, the backside of the second die 2210 of chip 2200 isused to define one or more interconnect layers on which power/groundlines are defined. For instance, in some embodiments, a firstinterconnect layer on the backside of the second die provides a firstset of alternating power and ground lines, while a second interconnectlayer on this backside provides another set of alternating power andground lines. These two sets of alternating power/ground lines form apower mesh (similar the meshes described above by reference to FIGS. 9and 10 ) as vias connect power lines in each set to power lines in theother set and ground lines in each set to ground lines in the other set.

The power/ground lines on such backside interconnect layer(s) arethicker and wider lines in some embodiments than the lines on the topinterconnect layers on the front side(s) of the first and second dies2205 and 2210. Gate stress is an undesirable side effect of having verythick power lines on the top interconnect layers on the front sides ofthe first and second dies. However, this is not an issue when placingthick (e.g., wide) power lines on the backside of IC dies. The thickerand wider power lines on the backside of the second die have lessresistance (suffer less signal degradation) and are ideal for supplyingadditional power signals to the core circuits on the first and seconddies. The circuits towards the center of a die can experience powersignal degradation due to the power consumption of the circuits that arecloser to the periphery of the die. Accordingly, in some embodiments,the power/ground lines on the backside of the second die is used in someembodiments to provide non-degraded power signals to the circuits thatare closer to the middle of the first and second dies.

Alternatively to, or conjunctively with, defining the power/ground lineson the backside of the second die 2210, clock lines and/or data-buslines are defined in some embodiments on the backside of the second die.Such clock lines and data-bus lines can be used to achieve analogousinterconnect architectures to those described above by reference toFIGS. 11-20B. As the backside interconnects can be thicker and wider,the clock lines and data-bus lines can enjoy the same benefits as thosedescribed above for the power lines that are defined on the backside ofthe second die 2210. In some embodiments, the interconnect line widthson the backside of the second die 2210 are in the range of 1-10 microns,while the interconnect line widths on the interconnect layers on thefront side of the first and second dies 2205 and 2210 are in the rangeof 1 microns or less.

FIG. 23 illustrates a manufacturing process 2300 that some embodimentsuse to produce the 3D chip 2200 of FIG. 22 . This figure will beexplained by reference to FIGS. 24-27 , which show two wafers 2405 and2410 at different stages of the process. Once cut, the two wafersproduce two stacked dies, such as dies 2205 and 2210. Even though theprocess 2300 of FIG. 23 cuts the wafers into dies after the wafers havebeen mounted and processed, the manufacturing process of otherembodiments performs the cutting operation at a different stage at leastfor one of the wafers. Specifically, some embodiments cut the firstwafer 2405 into several first dies that are each mounted on the secondwafer before the second wafer is cut into individual second dies.

As shown, the process 2300 starts (at 2305) by defining components(e.g., transistors) on the substrates of the first and second wafers2405 and 2410, and defining multiple interconnect layers above eachsubstrate to define interconnections that form micro-circuits (e.g.,gates) on each die. To define these components and interconnects on eachwafer, the process 2300 performs multiple IC fabrication operations(e.g., film deposition, patterning, doping, etc.) for each wafer in someembodiments. FIG. 24 illustrates the first and second wafers 2405 and2410 after several fabrication operations that have defined componentsand interconnects on these wafers. As shown, the fabrication operationsfor the second wafer 2410 defines several TSVs 2412 that traverse theinterconnect layers of the second wafer 2410 and penetrate a portion ofthis wafer's substrate 2416.

After the first and second wafers have been processed to define theircomponents and interconnects, the process 2300 face-to-face mounts (at2310) the first and second wafers 2205 and 2210 through a direct bondingprocess, such as a DBI process. FIG. 25 illustrates the first and secondwafers 2405 and 2410 after they have been face-to-face mounted through aDBI process. As shown, this DBI process creates a number of directbonded connections 2426 between the first and second wafers 2405 and2410.

Next, at 2315, the process 2300 performs a thinning operation on thebackside of the second wafer 2410 to remove a portion of this wafer'ssubstrate layer. As shown in FIG. 26 , this thinning operation exposesthe TSVs 2412 on the backside of the second wafer 2410. After thethinning operation, the process 2300 defines (at 2320) one or moreinterconnect layers 2430 the second wafer's backside. FIG. 27illustrates the first and second wafers 2405 and 2410 after interconnectlayers have been defined on the second wafer's backside.

These interconnect layers 2430 include one or more layers that allow the3D chip stack to electrically connect to the ball grid array. In someembodiments, the interconnect lines/pads on the backside of the thirdwafer also produce one or more redistribution layers (RDL layers) thatallow signals to be redistributed to different locations on thebackside. The interconnect layers 2430 on the backside of the second diein some embodiments also create the signal paths for defining one ormore system level circuits (e.g., power circuits, clock circuits, dataI/O signals, test circuits, etc.) for the circuits of the first andsecond dies. In some embodiments, the system level circuits are definedby circuit components (e.g., transistors, etc.) that are defined on thefront side of the second die. The process 2300 in some embodiments doesnot define interconnect layers on the backside of the second wafer tocreate the signal paths for the system level circuits, as it uses onlythe first and second dies' interconnect layers between their two facesfor establishing the system level signal paths.

After defining the interconnect layers on the backside of the secondwafer 2410, the process cuts (at 2325) the stacked wafers intoindividual chip stacks, with each chip stack include two stacked IC dies2205 and 2210. The process then mounts (at 2330) each chip stack on aball grid array and encapsulates the chip stack within one chip housing(e.g., by using a chip case). The process then ends.

In some embodiments, three or more IC dies are stacked to form a 3Dchip. FIG. 28 illustrates an example of a 3D chip 2800 with threestacked IC dies 2805, 2810 and 2815. In this example, the first andsecond dies 2805 and 2810 are face-to-face connected through directbonded connections (e.g., DBI connections), while the third and seconddies 2815 and 2810 are face-to-back connected (e.g., the face of thethird die 2815 is mounted on the back of the second die 2810). In someembodiments, the first and second dies 2805 and 2810 are the first andsecond dies shown in any of the FIGS. 1-20 .

In FIG. 28 , several TSVs 2822 are defined through the second die 2810.These TSVs electrically connect to interconnects/pads on the backside ofthe second die 2810, which connect to interconnects/pads on the topinterconnect layer of the third die 2815. The third die 2815 also has anumber of TSVs that connect signals on the front side of this die tointerconnects/pads on this die's backside. Through interconnects/pads,the third die's backside connects to a ball grid array 2840 that allowsthe 3D chip 2800 to mount on a printed circuit board.

In some embodiments, the third die 2815 includes system circuitry, suchas power circuits, clock circuits, data I/O circuits, test circuits,etc. The system circuitry of the third die 2815 in some embodimentssupplies system level signals (e.g., power signals, clock signals, dataI/O signals, test signals, etc.) to the circuits of the first and seconddies 2805 and 2810. In some embodiments, the system circuitry receivessome or all of the system level signals through the ball grid array 2840connected to the backside of the third die.

FIG. 29 illustrates another example of a 3D chip 2900 with more than twostacked IC dies. In this example, the 3D chip 2900 has four IC dies2905, 2910, 2915 and 2920. In this example, the first and second dies2905 and 2910 are face-to-face connected through direct bondedconnections (e.g., DBI connections), while the third and second dies2915 and 2910 are face-to-back connected (e.g., the face of the thirddie 2915 is mounted on the back of the second die 2910) and the fourthand third dies 2920 and 2915 are face-to-back connected (e.g., the faceof the fourth die 2920 is mounted on the back of the third die 2915). Insome embodiments, the first and second dies 2905 and 2910 are the firstand second dies shown in any of the FIGS. 1-20 .

In FIG. 29 , several TSVs 2922 are defined through the second, third andfourth die 2910, 2915 and 2920. These TSVs electrically connect tointerconnects/pads on the backside of these dies, which connect tointerconnects/pads on the top interconnect layer of the die below or theinterconnect layer below. Through interconnects/pads and TSVs, thesignals from outside of the chip are received from the ball grid array2940.

Other embodiments use other 3D chip stacking architectures. Forinstance, instead of face-to-back mounting the fourth and third dies2920 and 2915 in FIG. 29 , the 3D chip stack of another embodiment hasthese two dies face-to-face mounted, and the second and third dies 2910and 2915 back-to-back mounted. This arrangement would have the third andfourth dies 2915 and 2920 share a more tightly arranged set ofinterconnect layers on their front sides.

While the invention has been described with reference to numerousspecific details, one of ordinary skill in the art will recognize thatthe invention can be embodied in other specific forms without departingfrom the spirit of the invention. For instance, one of ordinary skillwill understand that even though several H-trees were described above asexample of clock distribution networks, other embodiments use othertypes of clock distribution networks. Also, in some embodiments, thestacked dies in a 3D chip share multiple different clock trees onmultiple shared interconnect layers in order to distribute multipledifferent clock signals (e.g., to distribute a different clock signalwith each different shared clock tree).

In the examples illustrated in FIG. 1-20 , a first IC die is shown to beface-to-face mounted with a second IC die. In other embodiments, thefirst IC die is face-to-face mounted with a passive interposer thatelectrically connects the die to circuits outside of the 3D chip or toother dies that are face-to-face mounted or back-to-face mounted on theinterposer. In some of these embodiments, the passive interposer caninclude the power, clock, and/or data bus interconnect linearchitectures that were described in FIGS. 1-20 for the second dies inthese examples. In other words, the interposer can provide theinterconnect layers for establishing the power, clock and data-bus linesof the 3D chip.

In some embodiments, the preferred wiring directions of the top layer ofthe interposer is orthogonal to the preferred wiring directions of thetop layer of the first die. This can be achieved by using similartechniques to those described above by reference to FIGS. 6-8 . Someembodiments place a passive interposer between two faces of two dies.Some embodiments use an interposer to allow a smaller die to connect toa bigger die.

Also, the 3D circuits and ICs of some embodiments have been described byreference to several 3D structures with vertically aligned IC dies.However, other embodiments are implemented with a myriad of other 3Dstructures. For example, in some embodiments, the 3D circuits are formedwith multiple smaller dies placed on a larger die or wafer. FIG. 30illustrates one such example. Specifically, it illustrates a 3D chip3000 that is formed by face-to-face mounting three smaller dies 3010 a-con a larger die 3005. All four dies are housed in one chip 3000 byhaving one side of this chip encapsulated by a cap 3020, and the otherside mounted on a micro-bump array 3025, which connects to a board 3030of a device 3035. Some embodiments are implemented in a 3D structurethat is formed by vertically stacking two sets of vertically stackedmulti-die structures.

The invention claimed is:
 1. A three-dimensional (3D) circuitcomprising: a first integrated circuit (IC) die comprising a firstsemiconductor substrate and a first set of interconnect layers definedon the first semiconductor substrate; and a second IC die comprising (i)a second semiconductor substrate, (ii) a second set of interconnectlayers defined on the second semiconductor substrate, and (iii) a clockcircuit defined on the second semiconductor substrate, the first andsecond IC dies face-to-face mounted so that the top interconnect layerof the first IC die is next to the top interconnect layer of the secondIC die, wherein at least one particular second-set interconnect layercomprises a plurality of clock interconnect segments to supply a clocksignal from the clock circuit to the first IC die through a plurality ofdirect-bonded connections between the first and second IC dies.
 2. The3D circuit of claim 1, wherein each IC die comprises a plurality oftransistors and a plurality of circuit elements formed byinterconnecting sets of transistors with a set of interconnect segmentson that die's set of interconnect layers, the plurality of circuitscomprising the clock circuit of the second IC die.
 3. The 3D circuit ofclaim 1, wherein a set of at least 1000 direct-bonded connections/mm²allow signals to traverse between the first and second IC dies, and eachof a subset of the connections carries a clock signal.
 4. The 3D circuitof claim 1, wherein a set of at least 10000 direct-bondedconnections/mm² allow signals to traverse between the first and secondIC dies, and each of a subset of the connections carries a clock signal.5. The 3D circuit of claim 1, wherein a set of at least 100000direct-bonded connections/mm² allow signals to traverse between thefirst and second IC dies, and each of a subset of the connectionscarries a clock signal.
 6. The 3D circuit of claim 1, wherein acenter-to-center pitch between two neighboring direct-bonded connectionsin each of a plurality of neighboring direct-bonded connections is 15 μmor less.
 7. The 3D circuit of claim 1, wherein a center-to-center pitchbetween two neighboring direct-bonded connections in each of a pluralityof neighboring direct-bonded connections is 5 μm or less.
 8. The 3Dcircuit of claim 1, wherein a center-to-center pitch between twoneighboring direct-bonded connections in each of a plurality ofneighboring direct-bonded connections is 0.5 μm or less.
 9. The 3Dcircuit of claim 1, wherein the first and second dies are face-to-facemounted through a direct bonding process, wherein at least a subset ofdirect bonded connections that supply a clock signal from the second dieto the first die are shorter than 1 micron.
 10. The 3D circuit of claim9, wherein the direct bonding process is a DBI (Direct BondInterconnect) process.
 11. An electronic device comprising: athree-dimensional (3D) circuit comprising: a first integrated circuit(IC) die comprising a first semiconductor substrate and a first set ofinterconnect layers defined on the first semiconductor substrate; and asecond IC die comprising (i) a second semiconductor substrate, (ii) asecond set of interconnect layers defined on the second semiconductorsubstrate, and (iii) a clock circuit defined on the second semiconductorsubstrate, the first and second IC dies face-to-face mounted so that thetop interconnect layer of the first IC die is next to the topinterconnect layer of the second IC die, wherein at least one particularsecond-set interconnect layer comprises a plurality of clockinterconnect segments to supply a clock signal from the clock circuit tothe first IC die through a plurality of direct-bonded connectionsbetween the first and second IC dies.
 12. The electronic device of claim11, wherein each IC die comprises a plurality of transistors and aplurality of circuit elements formed by interconnecting sets oftransistors with a set of interconnect segments on that die's set ofinterconnect layers, the plurality of circuits comprising the clockcircuit of the second IC die.
 13. The electronic device of claim 11,wherein a set of at least 1000 direct-bonded connections/mm² allowsignals to traverse between the first and second IC dies, and each of asubset of the connections carries a clock signal.
 14. The electronicdevice of claim 11, wherein a set of at least 10000 direct-bondedconnections/mm² allow signals to traverse between the first and secondIC dies, and each of a subset of the connections carries a clock signal.15. The electronic device of claim 11, wherein a set of at least 100000direct-bonded connections/mm² allow signals to traverse between thefirst and second IC dies, and each of a subset of the connectionscarries a clock signal.
 16. The electronic device of claim 11, wherein acenter-to-center pitch between two neighboring direct-bonded connectionsin each of a plurality of neighboring direct-bonded connections is 15 μmor less.
 17. The electronic device of claim 11, wherein acenter-to-center pitch between two neighboring direct-bonded connectionsin each of a plurality of neighboring direct-bonded connections is 5 μmor less.
 18. The electronic device of claim 11, wherein acenter-to-center pitch between two neighboring direct-bonded connectionsin each of a plurality of neighboring direct-bonded connections is 0.5μm or less.
 19. The electronic device of claim 11, wherein the first andsecond dies are face-to-face mounted through a direct bonding process,wherein at least a subset of direct bonded connections that supply aclock signal from the second die to the first die are shorter than 1micron.
 20. The electronic device of claim 11, wherein the plurality ofclock interconnect segments on the top interconnect layer of the seconddie is a first plurality of clock interconnect segments, the pluralityof direct-bonded connections connect the first plurality of clockinterconnect segments with a second plurality of interconnect segmentson a top interconnect layer of the first die, the second plurality ofinterconnect segments distributing the clock signal to circuits definedon the first IC die.
 21. The electronic device of claim 11, wherein thedirect-bonded connections are native connections that allow signals tospan first and second dies without any modifications.
 22. The electronicdevice of claim 21, wherein the native connections allow signals to spanfirst and second dies without passing through any intervening circuit.